Voltage controlled oscillators (VCOs) are widely used to generate a switching signal where some of the characteristics of the generated signal can also be controlled. Two types of VCOs are primarily used in high performance integrated circuits (ICs); inductor-capacitor (LC) oscillators and ring oscillators. LC oscillators can operate at very high frequencies and exhibit superior noise performance. Alternatively, ring oscillators occupy significantly smaller on-chip area and have a wider tuning range. Due to these advantages, ring oscillators have found widespread utilization in modern ICs.
A conventional ring oscillator consists of an odd number of inverters, wherein the output of the last inverter is fed back to the input of the first inverter. The delay provided by each inverter in the chain of inverters provides a phase shift to the switching signal. The sum of these individual delays (i.e., phase shifts) and the feedback from the last to the first inverter provides a phase shift of 2π, resulting in an oscillation. The frequency of this oscillation therefore depends upon the sum of the inverter delays within the chain.
The duty cycle of the generated switching signal is typically 50% for conventional ring oscillators when the PMOS (p-type metal-oxide semiconductor) and NMOS (n-type metal-oxide semiconductor) transistors within the inverters provide the same rise and fall transition slopes. The duty cycle of a ring oscillator can be changed by controlling the transition time of the inverters within the ring oscillator. Header and footer circuits are widely used to control the amount of current supplied to the PMOS and NMOS transistors within the ring oscillator inverter chain. It is desirable to be able to dynamically change the duty cycle of the ring oscillator, thereby providing a dynamically changing input switching signal to an on-chip voltage regulator. However, the header and foot circuits currently known in the art do not provide a means for dynamically changing the duty cycle of the pulse width modulator.
Accordingly, what is needed in the art is a pulse width modulator that provides a means for dynamic voltage scaling that can dynamically change the duty cycle of the input switching signal for on-chip voltage regulators. It is desirable to have a pulse width modulator that will enable high granularity dynamic voltage scaling (DVS) at runtime and substantially reduce the reaction time from milliseconds to nanoseconds.